iproute2: improved error messages

This patch improves many error messages as follows:
- For incorrect parameters, show the value of the offending parameter, rather than just say that it is incorrect
- Rephrased messages for clarity
- Rephrased to more `mainstream' english

Signed-off-by: Kees van Reeuwijk <reeuwijk@few.vu.nl>
diff --git a/ip/iplink_can.c b/ip/iplink_can.c
index c8af4bc..5674358 100644
--- a/ip/iplink_can.c
+++ b/ip/iplink_can.c
@@ -68,8 +68,8 @@
 		cm->flags |= flags;
 	} else if (strcmp(arg, "off") != 0) {
 		fprintf(stderr,
-			"Error: argument of \"%s\" must be \"on\" or \"off\"\n",
-			name);
+			"Error: argument of \"%s\" must be \"on\" or \"off\", not \"%s\"\n",
+			name, arg);
 		exit(-1);
 	}
 	cm->mask |= flags;
@@ -169,7 +169,7 @@
 			usage();
 			return -1;
 		} else {
-			fprintf(stderr, "can: what is \"%s\"?\n", *argv);
+			fprintf(stderr, "can: unknown option \"%s\"\n", *argv);
 			usage();
 			return -1;
 		}